Residential swimming pools contain a great deal of water (often 80,000 liters or more), and water must periodically be pumped out of these pools. A common reason for this is to use the removed water for backwashing the pool filter. As is well-known, when the filter is backwashed, water pumped from the pool is forced through the filter in a backwards direction (i.e., opposite to the normal direction of flow through the pool filter). The reverse flow of water cleans dirt and debris from the filter media (usually sand, but other materials such as diatomaceous earth can be used, in which case their pipe and valve arrangements can vary from those described which are for sand). The water used to clean the pool filter is now laden with dirt from the filter and in some cases with some of the filter media and is not normally directed back into the pool (since this would require that it be cleaned). Instead, the backwash water is directed out a backwash hose, typically to a storm sewer (usually by discharging the water onto a road). In some cases, the backwash water is sent to a sanitary sewer. In either case, the backwash water may contain an undesirable level of chlorine. Such chlorine can create environmental damage. Even the dirt load contained in the water may create undesirable consequences for the environment.
There may be other reasons (unrelated to backwashing) for removing water from a swimming pool. For example, the pool level may need to be lowered, e.g. due to excess rain, fall closing, cleaning of the pool, changing the liner, painting or repairing. However, this water may also contain chlorine and should be dechlorinated. For simplicity, all such water to be discharged will be referred to as “discharge water” or “backwash water” or “waste water”.